1. Field of the Invention
The present invention relates to a solution casting method and a polymer film produced by the solution casting method, and especially to a polymer film used for a polarizing filter and a liquid crystal display and a solution casting method as a producing method of the polymer film.
2. Description Related to the Prior Art
There are several sorts of polymer films for the optical use. In order to produce such a polymer film, a casting die is usually used to cast a dope onto a support, the cast dope is peeled as the polymer film from the support, and then wound as the polymer film after a drying process. This method is called a solution casting method and a representative method for producing the polymer film.
Because of the large requirement for increasing demands and decreasing the costs for the polymer film for the optical use, it is designated to increase the productivity of the polymer film, namely to increase the film production speed in the solution casting method. Further, in the optical use, the requirements for the higher function and the multi-function are large. Accordingly, it is necessary to make the thickness of the polymer film thinner.
In the above solution casting method, after the dope is peeled as the polymer film from the support, a tenter device for stretching the polymer film and regulating the width thereof is usually provided in a drying process, so as to improve the planarity, a mechanical strength, optical properties and the like.
After peeling from the polymer film from the support, there is an area in which the polymer film is transported toward the tenter device, and the area is called a transporting section, in which driven or non-driven rollers are provided to transport or support the film. In the transporting section, wrinkles, crumples, adhesion of surfaces that is caused with the wrinkles and crumples, curls in side edge portions and the like occur. These are problems on transporting the film in the transporting section. However, while a contact surface to the rollers has slight deformation, the quality of the film becomes lower and the precipitation of the plasticizer on the film surface and the like contaminate the surface of the rollers.
There are several methods for reducing the generation of the film defect in the transporting section. For example, a first roller to which the polymer film peeled from the support contact at first is disposed such that a peeled surface of the polymer film may contact thereto, and a second roller next to the first roller is disposed such that another surface (hereinafter exposed surface) of the polymer film may contact thereto. Further, after the contact to the second roller, the polymer film is dried (see, Japanese Patent Laid-Open Publication No. 2001-198933). Otherwise, at least in the side of the exposed surface, no rollers of the contacting type are used, but a non-contact type transporting device as an air floater, and otherwise, both side edges are held by a holding device such that nothing may contact to a production portion (see, Japanese Patent Laid-Open Publication No. 2001-277267).
Further, after the peeling, the curls formed in the side edges are slit off (see, Japanese Patent Laid-Open Publication No. H11-90942). In the transporting section, the rollers are arranged in zigzag and the rollers are heated to a predetermined temperature for a predetermined period (see, Japanese Patent Laid-Open Publication No. 2001-315147). Further, if it is designated to produce a film whose thickness is 20 μm to 80 μm, the rollers for transporting the film containing a predetermined amount of the remaining solvent satisfy conditions that the surface roughness Ry is at most 0.6 μm and the surface energy is from 70 mN/m to 100 mN/m (see, Japanese Patent Laid-Open Publication No. 2002-86474). Further, in the transporting section, a Vickers hardness of the rollers is in the range of 500 to 800 (see, Japanese Patent Laid-Open Publication No. 2002-292658).
However, in the above methods, it is hard to reduce the generation of the wrinkles and the crumples, the curl of the side edges and the pollution of the roller enough. Especially, when the thickness of the film to be produced is small, the effects are not large. For example, in the publication No. 2001-198933, the first roller to which the polymer film contacts at first after the peeling from the support contacts to the peeled surface. In this case, the content of the remaining solvent in the peeled surface is large and the peeled surface is soft. Therefore the contact to the first roller forms scratches on the peeled surface. Further, in the publication 2001-277267, the non-contact type transporting devices (such as an air floater type and the type for holding both side edges) are used. In this case, for example, when the thickness of the film to be produced is several dozen micrometers and the like or the content of the volatile solvent compounds is high, the film flatters too much and deforms to be corrugated. Thus the surface defect occur and the film is tore by the holding device.
Further, in the method for slitting off the side edges in which the curl is formed, as in the publication No. H11-90942, the curl is removed. However, the wrinkles and the folds are not reduced. In the publications No. 2002-86474 & 2002-292658, although the pollution of the rollers is reduced, the curls in the side edges are not reduced.